SZ124 Cluster failing

I have a customer with two SZ124, linked to a Cisco switch via 10Gb SFP+ interfaces. The cluster is configured to tunnel all WLAN traffic back to the controller on a few VLANs. We seem to only be able to keep one SZ online at a time. The one controller seems to drop offline as soon as we tag the VLANs up to the interface. Ideas?

The client VLANs are passed on each trunk interface, but I don't believe they're pushed all the way out to the edge switches. Each configured WLAN is tunneling all client traffic back to the controller